To tell someone how long an object is, you can't just say "as long as my arm" or "twice the length of my hand".
The length of people's arms are not the same.
Hands come in different sizes too.
That's why we have units of length.
Units of lengths are lengths that everyone agrees upon.
In the United States, the units of length are:
inches, feet, yards, and miles
In the rest of the world, the units of length include:
millimeters, centimeters, meters, and kilometers
These are the two systems of measurement used in the world.
The United States system is called the US customary system.
All other countries in the world use the metric system.
The units of length in the US customary system include inches, feet, yards, and miles.
The inch is the smallest unit in this list.
The length of this paper clip is 1 inch.
A foot is longer than an inch.
One foot is 12 inches.
A yard is longer than a foot.
One yard is 3 feet.
The mile is the largest unit.
One mile is 1,760 yards.
Each unit has a conversion factor that you just have to memorize. 🙈
A conversion factor is a number used to change a measurement from one unit to another.
Pause and memorize the conversion factors above.
Let's review.
The length of this paper clip is 1 inch.
A foot-long ruler is 12 times longer than the paper clip.
A yardstick is 3 times longer than a ruler. It measures 3 feet, or 1 yard.
We can use a yardstick to measure rolls of fabric.
Roads stretch for miles.
There are 1,760 yards, or 5,280 feet, in one mile.
The names of units are often abbreviated, or shortened, like this:
inch - in
foot/feet - ft
yard - yd
mile - mi
Tip: Even if you have more than one, never add an 's' to the abbreviations. So it's 1 ft, 2 ft, and never 3 'fts'.
This sandwich is 1 foot in length.
We can also say that its length is 12 inches.
Look at this fabric:
We've measured 1 yard of it.
We can also express 1 yard as 3 feet.
You can even say this fabric's length is 36 inches.
Let's explore the measurement equivalents in different units.
We use inches and feet when measuring common household objects.
The table below shows measurement equivalents between these two units.
To convert from feet to inches, just multiply the measurement in feet by 12.
To convert from inches to feet, just divide the measurement in inches by 12.
Let's practice converting:
How many inches are there in 15 feet?
We need to multiply 15 feet by 12.
15 × 12 = 180
15 feet = 180 inches ✅
Good job.
Let's try another one.
Convert 720 inches to feet.
We need to divide 720 inches by 12.
720 ÷ 12 = 60
720 inches = 60 feet ✅
We will learn more about conversions in the US customary system later.
Great work. 👏
Now, complete the practice, or continue to the next lesson on the metric system units of length used in the rest of the world. It's important to know both systems.